1. Field of the Invention
This invention relates to the construction of a machine rotor, in particular, it relates to the construction of rotors for high-speed, controlled-pole electric machines. For purposes of this application, the term high-speed machine shall mean a rotating machine wherein the rotational speed and the diameter of the rotating portion of the machine are such as, in their combination, to give rise to centrifugal forces of such magnitude acting upon the rotating portion of the machine as to require explicit consideration in the design and construction of the machine.
2. Description of Related Art
A controlled-pole electric machine includes a rotor and a stator, each held in position by a frame in such a fashion as to permit continuous mechanical rotation of the rotor relative to the stator.
Typically in controlled-pole machines, the adjacent surfaces of the rotor and the stator are of nominally cylindrical form, each of diameter similar to but not identical to the diameter of the other, the axes of these cylindrical surfaces being coaxial with each other such that the surface of the rotor passes in close fixed proximity to the surface of the stator during rotation of the rotor about this axis; the clearance region between said surfaces being otherwise known as the airgap. Controlled-pole machines may be of external rotor construction having the rotor outside the airgap such that the rotor surrounds the stator, or they may be of internal rotor construction having the rotor inside the airgap such that the stator surrounds the rotor.
A controlled-pole rotor includes an annular layer of permanent magnetic material, one surface of which, absent certain restraining materials which are a part of the invention described herein, is adjacent to and defines one boundary of the airgap; the magnetic material is usually mounted upon a core of high magnetic permeability, low eddy current and magnetic hysteresis loss material and attached to a rotating mechanical shaft. The permanent magnetic material in controlled-pole machines typically comprises a ferrite based, fired ceramic material having very low electrical conductance, high remanent magnetization, and a coercive force tailored to its controlled-pole usage.
A controlled-pole stator usually includes a structure of high magnetic permeability, low eddy current and magnetic hysteresis loss material having a number of slots running nominally axially along its airgap surface in which are inserted windings consisting of a number of electrical conductors; said electrical conductors often comprising, in part, several multi-turn coils, and being rotationally positioned and electrically interconnected so as to achieve the desired electrical, magnetic and mechanical characteristics. Certain of these windings, otherwise known as the main windings, are of such a number, positioning, and electrical connection as to be comparable in their function to the windings of a conventional fixed-pole electric machine. Certain other of these windings, otherwise known as the exciter windings and which are unique and essential to the controlled-pole machine, are of such a number, positioning, and electrical connection as to permit controlling the direction and magnitude of the magnetization of the rotor""s permanent magnetic material during operation of the machine.
During the rotor""s rotation, the permanent magnetic material of the rotor passes in close proximity to the stator, and in consequence of this motion and of the passage of an alternating current of appropriate magnitude through the exciter winding, the rotor""s permanent magnet layer is magnetized into a pattern of radially directed, alternating north and south magnetic poles, the effective rotational speed of said magnetic poles thereby being in synchronization with the alternating currents in the exciter winding irrespective of the rotational mechanical speed of the rotor. Subsequently, in the case of the machine""s functioning as a motor; these north and south magnetic poles in passing in close proximity to the main windings interact with the magnetic field produced by alternating currents in the main windings supplied from an external source thereby producing a rotor torque; or, in the case of the machine""s functioning as a generator, an alternating pattern of north and south magnetic poles is produced in the rotor""s magnetic layer in a manner similar to that described for the case of the motor, and an externally supplied torque causes the rotor""s north and south magnetic poles to pass in close proximity to the main windings thereby inducing an alternating current in the main windings and thence to any externally connected electrical load, said main winding currents being synchronous with the currents in the exciter windings irrespective of the rotational mechanical speed of the rotor. In both cases, the alternating current in the exciter windings and the alternating currents in the main windings are caused to be synchronized with each other through the controlling of the rotor""s magnetic poles and are independent of the speed of mechanical rotation of the rotor, thus permitting the delivery of continuous, smooth torque independent of speed in the case of the machine""s functioning as a synchronous motor, or the continuous delivery of a constant frequency alternating electrical current independent of speed in the case of the machine""s functioning as a synchronous alternator.
It is common to construct the rotor""s permanent magnet layer from a number of individual pieces of magnetic material assembled in mosaic-like fashion in one or more annular layers to form the desired composite magnetic cylinder, the individual pieces typically being bonded to each other and to the underlying high permeability core by means of a bonding agent such as a high strength epoxy.
The operation of a rotor of this form of construction as a controlled-pole rotor creates several conditions which, in a manner or degree not material to operation of a fixed-pole machine and not obvious to those skilled in the art of fixed-pole or controlled-pole machines, present opportunity for inventive enhancement or which rise to the level of becoming problematic and compelling of inventive solution. This is particularly true in a machine of internal rotor construction having the rotor inside the airgap such that the stator surrounds the rotor,
One such instance arises from the substantial thickness of the annular magnetic material layer of the internal rotor which results when two adjacent cylindrical surface layers of the magnetic material of the rotor have significantly different diameters and, as a consequence, significantly different surface areas. In the controlled-pole machine, it is common for these respective cylindrical surface areas to differ by twenty percent or more. This condition also obtains for the cylindrical surface of the high permeability core of the stator adjacent to the airgap and for the cylindrical surface of the high permeability core of the rotor underlying the magnetic material layer. During operation of the controlled-pole machine, substantially all the magnetic flux produced within the stator core by the currents in the various stator windings and the magnetic flux produced by the rotor""s layer of magnetic material traverses and is contained within the high permeability core of the stator, the airgap, the rotor""s magnetic material layer, and the high permeability core of the rotor. The combination of all these circumstances, that is substantially constant total quantity of magnetic flux traversing said regions, and said surfaces varying in area according to their distance from the axis of rotation, results in a condition wherein the magnetic flux density varies throughout the volume of the magnetic material layer of the rotor according to the distance from the axis of rotation.
Additionally, while the magnetic flux produced by currents in the main winding typically traverses the magnetic layer in a radial or substantially radial direction, in some embodiments, the magnetic flux produced by currents in the exciter winding can depart significantly from a radial direction in its traversal of the interior regions of the rotor""s layer of magnetic material. This can cause additional variations in flux density vs. depth into the magnetic layer over those described above for the magnetic fields produced by the currents in the exciter windings, thereby causing variation in the remagnetization effectiveness of the exciter winding currents with depth in the magnet layer.
Also, the performance objectives of a controlled-pole rotor can require an annular magnet layer that is thicker in the direction of the magnetization axis of the ferrite than current manufacturing techniques are able to achieve, thereby requiring the magnet layer be constructed as a composite layer of magnet material comprising two or more individual annular magnet layers. This combination of radially varying magnetic field densities and multiple layers of magnet material presents an inventive opportunity to enhance operation of the controlled-pole machine by employing magnets having magnetic properties tailored to each of the different layers.
Another area permitting of inventive attention in the construction of the controlled-pole internal rotor arises from the circumstance that during operation, a machine rotor experiences centrifugal forces which act on the components of the rotor in a manner which will cause them to depart from the rotor and their circular path of motion if there are not sufficient restraining centripetal forces present arising out of the properties of the materials and construction of the rotor so as to completely counter said centrifugal forces and hold the components in place. Additional structural devices and/or alternative materials and construction methods can be required to assure safe and reliable high-speed operation of a rotating machine. Several operating phenomena and functional requirements unique to controlled-pole machines have important significance to the design of the high-speed, controlled-pole rotor, and in particular, to the internal rotor controlled-pole machine.
In a controlled-pole rotor of internal rotor construction, the permanent magnet layer lies at the outer periphery of the rotor where centrifugal forces are greatest and where, in the absence of any specially added separate restraining mechanism(s), the only intrinsic mechanisms providing the centripetal forces necessary to counter said centrifugal forces and prevent separation of the rotor components are those provided by the bonding agents, by friction, and by such magnetic forces as may from time to time be present. Several phenomena can occur during operation of the controlled-pole rotor which can further limit the effectiveness of these intrinsic restraining mechanisms.
First, the ferrite ceramics typically used in the controlled-pole machine commonly have fissile properties arising from the process used in their manufacture and which can result in the magnets fracturing or delaminating along planes that are nominally parallel to the air gap. Also, this potential for delamination can be aggravated by certain aspects of the thermal stresses discussed further below. In the internal rotor machine in particular, if delamination should occur, the vulnerability of the magnets to the previously discussed centrifugal forces is increased. Consequently, construction of the controlled-pole motor must accommodate the potential fissile property of its magnets by means of identifying and eliminating potentially problematic magnets from its construction, and/or by incorporation of a restraining mechanism that will operate effectively on delaminated fragments.
Second, in a controlled-pole machine, the rotor""s magnetic layer can experience a greater and significantly more rapid increase in temperature than is experienced by conventional fixed-pole permanent magnet machines of otherwise equivalent design. This is a consequence of hysteresis loss occurring in the magnetic layer of the controlled-pole rotor whenever it is being remagnetized under controlled-pole or start-up operation and which manifests itself as heat being produced within the body of the magnetic material. The magnetic material employed in controlled-pole machines is typically a ferrite based ceramic having a coefficient of thermal expansion markedly different from that of the underlying metal core; such ceramics also typically have significantly lower coefficients of thermal conductivity than the core. All these conditions combine uniquely in the controlled-pole machine to produce the potential for there being substantial shear forces between the magnetic material and the core, thereby increasing internal bond stresses and reducing the bonds"" operating limits. Consequently, construction of the controlled-pole motor must accommodate this differential thermal expansion to avert the large shear forces which otherwise can result between the magnetic layer and the core, and/or incorporate additional mechanical devices to augment the centripetal restraining forces provided by the epoxy bonds.
These phenomena suggest the incorporation of additional mechanical device(s) around the outer surface of the magnet layer to augment the centripetal restraining forces already present, similar devices having been incorporated in fixed-pole machines. However, in a controlled-pole machine, such a device must not only take into account the thermo-mechanical issues peculiar to controlled-pole operation as discussed above, but any object placed around or otherwise in close proximity to the magnetic material for structural purposes must also take into account certain electromagnetic issues peculiar to electric machine operation in general or controlled-pole machine operation in particular.
First, the combination of the thickness and the magnetic properties of any layer interposed between the magnetic layer and the airgap for structural purposes must be such as to not materially reduce the radially directed magnetic flux emanating from the permanent magnet layer and linking with the main stator windings across the airgap.
Second, the magnetic fields produced by the controlling alternating currents in the stator exciter winding can induce eddy currents in any component which might be added to the controlled-pole rotor to provide mechanical constraint to the magnets and which is also electrically conductive thereby resulting in resistive heating losses therein, such eddy current losses being the reason machine cores are typically made of laminated steel or non-conductive, magnetically soft ferrite materials, high magnetic permeability also being a desirable property of these cores.
Third, eddy currents also produce magnetic fields which are in opposition to the magnetic field inducing the eddy current. It being the purpose of the magnetic field produced by the exciter winding currents to remagnetize the rotor""s permanent magnetic layer, any such opposing eddy current field which might occur in any material interposed between the magnetic layer and the airgap for structural purposes can reduce the strength of the remagnetizing field within the magnetic material thereby impeding controlled-pole operation.
Fourth, the presence of winding slots in the cylindrical surface of the stator results in a condition where rotation of the rotor causes any given spot on the rotor""s cylindrical surface to pass a substantial number of slot edges during each rotation of the rotor. During operation of the controlled-pole machine, the magnetic fields produced in the stator by currents in the main windings as well as those produced in the stator by the rotor""s permanent magnetic material are, when traversing the slot region of the stator core, primarily concentrated in the inter-slot, high permeability portion of the stator core. This results in there being, at any given spot on the rotor""s airgap surface, an abrupt and substantial change in the magnitude of the magnetic field as that spot passes each slot edge. Some embodiments of the controlled-pole internal rotor machine may include as manifestations of the inventions described herein, a thin layer of material adjacent to the airgap and overlying the remagnetizable permanent magnet material layer to provide restraint for the magnetic material against centrifugal forces. If said restraining material layer is magnetically permeable, as would be the case for a material such as high tensile strength steel piano wire, there can result, due to said differences in field strengths between the inter-slot, high permeability core region and the slot opening, significant magnetic field components tangential to the rotor""s cylindrical surface, further, said tangential components can experience a reversal in direction at each passage across a slot edge. If said restraining material layer further has properties of remanence and coercive force, as would again be typical of a material such as high tensile strength steel piano wire, there can result magnetic hysteresis loss within said restraining material layer at each crossing of a slot edge and corresponding reversal of magnetic field, there being two such edge crossings for every slot for every rotation of the rotor. In an otherwise typical controlled-pole machine where consideration is not given to this effect, the cumulative hysteresis loss occurring within the restraining wire layer of a rotor of such construction can be more than one percent of the total power of the machine. As a consequence, the design and construction of any restraining mechanism interposed between the magnetic layer and the airgap must be limited in thickness, electrical conductivity, and in magnetic permeability or remanence while offering high tensile strength and compatibility in thermal expansion with the other components of the rotor.
Due to the relative geometric proportions of the slots, the airgap diameter, and the thickness of the remagnetizable permanent magnet layer in typical controlled-pole machines, and specifically in controlled-pole machines having two or more layers of magnetic material, the abrupt changes in magnetic field due to the inter-slot high permeability stator core and the open slots diminish as the fields penetrates deeper into the permanent magnetic material layer, becoming negligible at the interface between layers of permanent magnetic material. In this region, however, there can be localized abrupt changes in the magnetic field arising from gaps between individual blocks of magnetic material and the mosaic tile type construction of the layer, and it can then, by contrast with the case at the surface of the rotor, be beneficial to employ a magnetically permeable material for the restraining material overlying any inner layers of magnetic material in order to smooth the overall distribution of the magnetic field.
The present invention is an apparatus which overcomes the deficiencies in the prior art. The present invention is a rotor for use in a high speed controlled-pole electric machine. A rotor for use in a high speed controlled-pole electric machine in accordance with the inventive arrangements provides advantages over all current rotors now used and provides a novel and nonobvious construction of a rotor for use in a high speed controlled-pole electric machine. A rotor for use in a high speed controlled-pole electric machine can include a rotor core having an exterior surface; a first layer of remagnetizable permanent magnet material positioned about the rotor core exterior surface; and a first layer of high strength material binding the first layer of remagnetizable magnetic material to the rotor core exterior surface. Notably, the rotor core can be a laminated steel rotor core.
Where used in a smaller controlled-pole electric machine, only a single layer of remagnetizable magnetic material may be necessary. However, where used in a larger controlled-pole electric machine, including several layers of remagnetizable permanent magnet material may be advantageous. In particular, the present invention can include one or more layers of remagnetizable permanent magnet material disposed about the first layer of remagnetizable magnetic material and the rotor core exterior surface. Moreover, the present invention can include an additional layer of high strength material binding each successive layer of remagnetizable magnetic material to the layer of remagnetizable magnetic material beneath it. A general embodiment of this concept would cause successive composite layers comprising a layer of remagnetizable magnetic material followed by a layer of high strength material to be placed over underlying similar composite layers until the desired overall thickness of magnetic material is obtained.
The individual layers of remagnetizable permanent magnetic material in a multi-layer structure can each comprise a remagnetizable permanent magnet material differing somewhat in its magnetic properties from layer to layer in order to compensate for the variations which can occur radially throughout the volume of the magnetic material in the flux densities of the magnetic fields.
Further, each layer of remagnetizable material can comprise a plurality of magnetic blocks positioned adjacent to one another. Each magnetic block can have a separation gap between each adjacent magnetic block. The separation gap can be calculated to allow for a thermal expansion of each magnetic block relative to the adjacent magnetic blocks.
The rotor can also include in each layer of remagnetizable magnetic material an adhesive layer disposed on each magnetic block surface facing the rotor core surface or the surface of a magnetic block beside it. The use of a thermally conductive, pliant adhesive in the construction of the present invention can improve the thermal conductivity between the layer(s) of remagnetizable magnetic material and the rotor core. In addition, because the high strength layer provides the necessary binding force, the adhesive layer need not be chosen only for its bonding strength characteristics. Rather, the adhesive can be chosen for its low viscosity and wetting ability during application and for its thermal conductivity, pliability and resistance to thermal aging once set.
The high strength material used to bind each of the layers of remagnetizable magnetic material preferably should exhibit strength properties similar to common music wire. Music wire typically has a tensile strength ranging from 225,000 PSI to over 400,000 PSI. Moreover, the high strength material used to bind the inner layers of remagnetizable magnetic material preferably should exhibit the magnetic properties of common music wire. Specifically, common music wire has the magnetic properties of a low coercive permanent magnet. The magnetic properties of each of the inner layers of high strength material can serve to improve the magnetic path of the rotor flux by bridging gaps between separate magnetic blocks in each layer of remagnetizable magnetic material. In the preferred embodiment, the coercivity of the first layer of high strength wire typically can be in the order of 50 oersteds. In addition, the residual induction can be approximately one tesla, or 10,000 gauss.
Reduction of slot edge magnetic field change induced hysteresis losses in and due to the outermost high strength material binding layer adjacent to the stator is of importance. This can be accomplished by employing a non-conductive non-magnetic high strength fiber of a material such as carbon, for instance, for the outer layer of high strength material. Alternatively, the outer layer can be made effectively non-electrically conductive by employing a suitably high electrical resistance, non-magnetic, high strength metallic wire consisting of, for instance, a material such as certain stainless steel alloys, each strand of said metallic wire being placed circumferentially on the rotor and in such a fashion as to leave a space between each adjacent strand of wire such that there is no axially directed electrical path between the adjacent strands. Thus, it is an advantage of the present invention that the use of a high resistance or a non-conductive non-magnetic layer of high strength material can reduce losses in the outermost binding material layer caused by eddy currents and by flux reversals caused by slot-edge induced rapid flux changes in the air gap.
A method of constructing a rotor for use in an internal rotor, high speed, controlled-pole electric machine can comprise the steps of: positioning a plurality of magnetic blocks about a rotor core exterior surface, the magnetic blocks forming a first layer of remagnetizable magnetic material; and, binding the first layer of remagnetizable magnetic material to the rotor core exterior surface using a first layer of high strength wire or fiber. The method can further comprise the steps of: positioning a plurality of magnetic blocks about the first layer of remagnetizable material, the magnetic blocks forming a second layer of remagnetizable magnetic material; and, binding the second layer of remagnetizable magnetic material to the first layer of remagnetizable magnetic material and the rotor core exterior surface using a second layer of non-conductive non-magnetic high strength fiber or wire. One skilled in the art, however, will appreciate that, while only a single layer of remagnetizable magnetic material may be necessary in smaller controlled-pole machines, and that larger machines may require two or more layers may be required, the binding for the outermost layer should consist of non-conductive, non-magnetic wire or fiber, and that the binding of any intermediate layers should consist of magnetic high strength wire.
The positioning step can further include: calculating a separation gap between each adjacent magnetic block to allow for a thermal expansion of each magnetic block; and, separating each magnetic block by the calculated distance. In one embodiment, the separation can be in the order of 0.002 inches for each inch of magnetic block length. The method can further include the step of applying a thermally-conductive pliant adhesive layer on each magnetic block surface facing the rotor core exterior surface or facing the surface of an adjacent magnetic block, the adhesive layer holding each magnetic block in place prior to the binding step.
Finally, the binding step can further include: pretensioning one or more parallel strand(s) of high strength fiber or wire filaments to a tenseness substantially greater than that required to counter the maximum centripetal force anticipated to be experienced by the layer of remagnetizable magnetic material being bound when rotating the rotor in a high-speed controlled-pole electric machine; securing one end of the high strength fiber or wire(s) to one end of the rotor; and, wrapping the filament(s) about the layer of remagnetizable magnetic material, securing a second end of the filament(s) to a second end of the rotor, thereby forming a plurality of parallel strands. In the case where high strength metal wire is used for the binding layer, having a small but distinct space between each parallel strand of the high strength wire greatly increases the electrical resistance of the layer in the axial direction thereby reducing eddy current losses to an acceptable level. Further, winding multiple filaments side by side in an in-hand fashion constituting a single lay, preserving a space between each wire if of a conductive wire, and similarly wrapping the lay of filaments about the layer of remagnetizable magnetic material and securing the second end of the lay in a similar fashion, reduces assembly time.